Takahiko Aoyama

Enlarged longitudinal dose profiles in cone-beam CT and the need for modified dosimetry

In order to examine phantom length necessary to assess radiation dose delivered to patients in cone-beam CT with an enlarged beamwidth, we measured dose profiles in cylindrical phantoms of sufficient length using a prototype 256-slice CT-scanner developed at our institute. Dose profiles parallel to the rotation axis were measured at the central and peripheral positions in PMMA (polymethylmethacrylate) phantoms of 160 or 320 mm diameter and 900 mm length. For practical application, we joined unit cylinders (150 mm long) together to provide phantoms of 900 mm length. Dose profiles were measured with a pin photodiode sensor having a sensitive region of approximately 2.8 x 2.8 mm2 and 2.7 mm thickness. Beamwidths of the scanner were varied from 20 to 138 mm. Dose profile integrals (DPI) were calculated using the measured dose profiles for various beamwidths and integration ranges. For the body phantom (320-mm-diam phantom), 76% of the DPI was represented for a 20 mm beamwidth and 60% was represented for a 138 mm beamwidth if dose profiles were integrated over a 100 mm range, while more than 90% of the DPI was represented for beamwidths between 20 and 138 mm if integration was carried out over a 300 mm range. The phantom length and integration range for dosimetry of cone-beam CT needed to be more than 300 mm to represent more than 90% of the DPI for the body phantom with the beamwidth of more than 20 mm. Although we reached this conclusion using the prototype 256-slice CT-scanner, it may be applied to other multislice CT-scanners as well.

Radiation dose evaluation in 64-slice CT examinations with adult and paediatric anthropomorphic phantoms

The objective of this study was to evaluate the organ dose and effective dose to patients undergoing routine adult and paediatric CT examinations with 64-slice CT scanners and to compare the doses with those from 4-, 8- and 16-multislice CT scanners. Patient doses were measured with small (<7 mm wide) silicon photodiode dosemeters (34 in total), which were implanted at various tissue and organ positions within adult and 6-year-old child anthropomorphic phantoms. Output signals from photodiode dosemeters were read on a personal computer, from which organ and effective doses were computed. For the adult phantom, organ doses (for organs within the scan range) and effective doses were 8-35 mGy and 7-18 mSv, respectively, for chest CT, and 12-33 mGy and 10-21 mSv, respectively, for abdominopelvic CT. For the paediatric phantom, organ and effective doses were 4-17 mGy and 3-7 mSv, respectively, for chest CT, and 5-14 mGy and 3-9 mSv, respectively, for abdominopelvic CT. Doses to organs at the boundaries of the scan length were higher for 64-slice CT scanners using large beam widths and/or a large pitch because of the larger extent of over-ranging. The CT dose index (CTDI(vol)), dose-length product (DLP) and the effective dose values using 64-slice CT for the adult and paediatric phantoms were the same as those obtained using 4-, 8- and 16-slice CT. Conversion factors of DLP to the effective dose by International Commission on Radiological Protection 103 were 0.024 mSvmGy(-1)cm(-1) and 0.019 mSvmGy(-1)cm(-1) for adult chest and abdominopelvic CT scans, respectively.

An in-phantom dosimetry system using pin silicon photodiode radiation sensors for measuring organ doses in x-ray CT and other diagnostic radiology

A dosimetry system using commercially available pin silicon photodiodes as the sensor is evaluated for in-phantom dose measurements in x-ray CT and other diagnostic radiology. System sensitivity measured as a function of the effective energy of x rays was between 0.37 and 0.49 V/mGy at an effective energy range between 23.5 and 72 keV. The minimum detectable organ dose with 25% uncertainty was estimated to be 0.02 mGy. The excellent output linearity was found over a dose range from 0.03 to more than 10 mGy with flat dose rate response of system sensitivity up to 35 mGy s(-1), though the sensitivity indicated some energy dependence across the diagnostic energy range with a maximum of about 10%/10 keV. Since angular dependence of the sensitivity of the photodiode sensor was found to be small enough it would induce negligible dose error. Dose profile measurement along the axis of a thoracic phantom undergoing CT chest examination indicated the reliability of dose values over a range of two orders of magnitude from less than 0.2 to 12 mGy. The present dosimetry system having advantages of high sensitivity with immediate readout of dose values, low cost, and easy construction would widely be used as an alternative to TLD dosimeters for organ and skin dose measurements in CT and other diagnostic radiology.

Generalized gas gain formula for proportional counters

Theoretical consideration of gas amplification in proportional counters has indicated that α/N, the ratio of the first Townsend coefficient to the gas density, is generally expressed in the form, α/N = KSmexp(− L/S1−m), where S = E/N is the ratio of the electric field strength to the gas density, and K, L and m (0 ⩽ m ⩽ 1) are constants characteristic of the gas. When m = 0 the α/N formula reduces to the analytical form used by Williams and Sara, and when m = 1 it reduces to that assumed by Diethorn. A gas gain formula derived from the α/N formula fitted the experimental gas gain data by Charles for a mixture of Ar + 10% CH4 and by Hendricks for a mixture of Xe + 5% CO2 when m = 12 was assumed. Theoretical justification for m = 12 was confirmed from the data of the total cross section for electrons in these gases and the characteristic energy of electrons in an electric field.

Detection of extremely low level radioactivity with imaging plate

Imaging plate (IP) is useful for in-situ detection and distribution measurements of extremely low-level radioactivity. Exposure of IP for several hours makes it possible to detect down to about 10−4 Bq in a spot with an area of around 1 mm2 or less. Methods to determine radionuclide species by only a single exposure are proposed; the several sheets lamination method and the several times successive read-out method. Although the latent image degraded with time after exposure, simultaneous exposure of α-, low-energy β- and high-energy β-calibration sources together with the specimen led to a quantitative analysis.

Radiation dose evaluation in tomosynthesis and C-arm cone-beam CT examinations with an anthropomorphic phantom

PURPOSE The objective of this study was to evaluate organ dose and the effective dose to patients undergoing tomosynthesis (TS) and C-arm cone-beam computed tomography (CBCT) examinations and to compare the doses to those in multidetector CT (MDCT) scans. METHODS Patient doses were measured with small sized silicon-photodiode dosimeters, 48 in number, which were implanted at various tissue and organ positions within an anthropomorphic phantom. Output signals from photodiode dosimeters were read out on a personal computer, from which organ and effective doses were computed. The doses in head, chest, abdomen, and hip-joint TS, and in head and abdomen C-arm CBCT were evaluated for routine protocols on Shimadzu TS and C-arm CBCT systems, and the doses in MDCT with the same scan regions as in TS and CBCT were on Toshiba 64-detector-row CT scanners. RESULTS In TS examination of the head, chest, abdomen, and hip-joint, organ doses for organs within scan ranges were 1-4 mGy, and effective doses were 0.07 mSv for the head scan and around 1 mSv for other scans. In C-arm CBCT examinations of the head and abdomen, organ doses within scan range were 2-37 mGy, and effective doses were 1.2 mSv for the head scan and 4-5 mSv for abdominal scans. Effective doses in TS examinations were approximately a factor of 10 lower, while the doses in CBCT examinations were nearly the same level, compared to the doses in the corresponding MDCT examinations. CONCLUSIONS TS examinations with low doses and excellent resolutions in coronal images compared to recent MDCT would widely be used in tomographic examinations of the chest, abdomen, pelvis, skeletal-joints, and knee instead of MDCT examinations with significantly high doses. Since patient dose in C-arm CBCT was nearly the same level as that in recent MDCT, the same consideration for high radiation dose would be required for the use of CBCT.

Energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523

Abstract The energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 was examined for the neutron energy range between 1.2 and 14 MeV. Pulse height spectra measured with monoenergetic neutrons evidenced that they were characterized by either a single peak with a shoulder or by double peaks. A Monte Carlo simulation of the neutron behavior in the detector reproduced the spectra by considering the nonlinear light yield against recoil proton energy. Detection efficiencies ranging from 10% for 1.2 MeV to 0.6% for 14 MeV neutrons were obtained with a 12.7 cm diameter × 7.6 cm long scintillator. A high background rejection with a ratio of 2200 to 1 obtained without using any γ-shielding makes the present neutron spectrometer attractive for the application to low-level environmental neutron measurements.

A neutron detector using silicon PIN photodiodes for personal neutron dosimetry

Abstract A highly sensitive and small sized neutron detector was devised for use as a personal neutron dosimeter using a silicon PIN photodiode and a gadolinium-foil converter. A detection efficiency of 5.6% was obtained for thermal neutrons by detecting internal-conversion electrons emitted from neutron-captured gadolinium. The γ-ray component contained in the output pulses was cancelled almost completely by using a twin photodiode with a tin foil instead of the gadolinium-foil converter. A low detection limit for neutron flux density of 3.1 × 10 2 cm −2 s −1 was obtained for thermal to 10 keV neutrons with a measurement time of 3 s under γ-ray background with a dose rate of 25 μSv/h.

A depth-dose measuring device using a multichannel scintillating fiber array for electron beam therapy

The development of a new depth-dose measuring device for electron beam therapy is described. The device employs plastic scintillating fiber detectors inserted in a polymethylmethacrylate (PMMA) phantom in line along an incident electron beam. Output photons from a fiber, the number of which is proportional to the absorbed dose at each depth of the phantom, were converted to an electric signal with a photodiode. Each signal from the photodiode was transmitted to a personal computer through a multichannel analog-digital converter, and was processed to draw a depth-dose curve on the computer display. A depth-dose curve could be obtained in a measuring time of 5 s for each incident electron beam with an energy range between 4 and 21 MeV. The mean electron energies estimated using the curves and the depth-scaling factor for PMMA were consistent with those obtained from conventional depth-dose measurements using an ion chamber and a water phantom. The newly developed system, being simple and not time consuming, could be used routinely for quality assurance purposes in electron beam therapy.

Patient radiation dose in prospectively gated axial CT coronary angiography and retrospectively gated helical technique with a 320‐detector row CT scanner

PURPOSE The aim of this study was to evaluate radiation dose to patients undergoing computed tomography coronary angiography (CTCA) for prospectively gated axial (PGA) technique and retrospectively gated helical (RGH) technique. METHODS Radiation doses were measured for a 320-detector row CT scanner (Toshiba Aquilion ONE) using small sized silicon-photodiode dosimeters, which were implanted at various tissue and organ positions within an anthropomorphic phantom for a standard Japanese adult male. Output signals from photodiode dosimeters were read out on a personal computer, from which organ and effective doses were computed according to guidelines published in the International Commission on Radiological Protection Publication 103. RESULTS Organs that received high doses were breast, followed by lung, esophagus, and liver. Breast doses obtained with PGA technique and a phase window width of 16% at a simulated heart rate of 60 beats per minute were 13 mGy compared to 53 mGy with RGH technique using electrocardiographically dependent dose modulation at the same phase window width as that in PGA technique. Effective doses obtained in this case were 4.7 and 20 mSv for the PGA and RGH techniques, respectively. Conversion factors of dose length product to the effective dose in PGA and RGH were 0.022 and 0.025 mSv mGy(-1) cm(-1) with a scan length of 140 mm. CONCLUSIONS CTCA performed with PGA technique provided a substantial effective dose reduction, i.e., 70%-76%, compared to RGH technique using the dose modulation at the same phase windows as those in PGA technique. Though radiation doses in CTCA with RGH technique were the same level as, or some higher than, those in conventional coronary angiography (CCA), the use of PGA technique reduced organ and effective doses to levels less than CCA except for breast dose.